Adhesion temperature dependence

Table 1 provides the approximate load bearing capabiUties of various adhesive types. Because the load-bearing capabiUties of an adhesive are dependent upon the adherend material, the loading rate, temperature, and design of the adhesive joint, wide ranges of performance are Hsted. 

Free mono- and multilayer films may be adhesive- or extmsion-bonded in the laminating process. The bonding adhesive may be water- or solvent-based. Alternatively, a temperature-dependent polymer-based adhesive without solvent may be heated and set by cooling. In extmsion lamination, a film of a thermoplastic such as polyethylene is extmded as a bond between the two flat materials, which are brought together between a chilled and backup roU. 

Micro-mechanical processes that control the adhesion and fracture of elastomeric polymers occur at two different size scales. On the size scale of the chain the failure is by breakage of Van der Waals attraction, chain pull-out or by chain scission. The viscoelastic deformation in which most of the energy is dissipated occurs at a larger size scale but is controlled by the processes that occur on the scale of a chain. The situation is, in principle, very similar to that of glassy polymers except that crack growth rate and temperature dependence of the micromechanical processes are very important. 

A second general criterion for pressure sensitivity is that the glass transition temperature of the adhesive be below the use temperature, which is usually room temperature. Broadly speaking, the To will be about 30-70°C below room temperature, depending on the base polymer and any added modifiers. 

The coating technique starts by applying a solvent-based adhesive on to a previously pretreated metal substrate. The item is then preheated to 200-250°C, the exact time and temperature depending on the metal thickness. It is then dipped in the plastisol which partly gels owing to the... 

The Kirkendall effect (8) is time and temperature dependent, and with some metal couples, it takes place even at room temperature. For instance, adhesion of solder to gold is damaged by heating to about 150°C for about 5 minutes, due to the formation of Kirkendall voids. Naturally, the formation of Kirkendall voids is accelerated by increased temperature and dwelling time. 

Fig. 22 Images and data representing development and application of DLS on a chip a one iteration in the design of a microfluidic DLS fabricated from aluminum with the surface anodized black to reduce surface reflections b image of a microfluidic chip that integrates polymer synthesis with DLS. The machined channels have been covered by a Kapton sheet fixed with adhesive c data for temperature depended micelle formation of polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer (Pluronic P85) at 2% by volume in water. (Derived from [106] with permission)...

Moon SH, Chiche A, Forster AM, Zhang WH, Stafford CM (2005) Evaluation of temperature-dependent adhesive performance via combinatorial probe tack measurements. Rev Sci lustrum 76 062210... 

Modern motor oil provides an example of some of the ways in which a number of colloidal and interfacial considerations come into play adhesion and lubrication, detergency, dispersion and suspension stabilization, foam inhibition, and viscosity and its temperature dependence. In addition to providing lubrication, a motor oil is expected to prevent corrosion and aid engine cooling and cleaning. Table 8.1 shows how a number of additives are blended in to help the oil achieve these functions [491]. 

Figure 3. Temperature dependence of rate of wet-strength loss caused by dry heat in joints of yellow birch and Douglas-fir with acidic and nonacidic adhesives (48).

Thermally responsive polymers, such as poly( V-isopropyl acrylamide) (NI-PAm), have also been studied extensively for applications related to those previously discussed [112], De las Heras et al. described the synthesis and patterning of NIPAm brushes on SAMs and their subsequent performance during temperature-dependent adhesion assays of BSA and Streptococcus mutans (Fig. 7). The authors employed p.CP to pattern features of hydrophobic hexadecanethiol and backfilled the surface with an initiator-functionalized alkanethiol. Polymer brushes were grown via surface-initiated atom transfer radical polymerization (ATRP). FITC-BSA was then... 

Fig. 7 (a) Growth of temperature-dependent, patterned polymer brushes on SAMs on gold surfaces. Images show adhesion of (b) FITC-BSA after incubation at 37°C and rinse at 12°C (c) S. mutans after incubation at 4°C for 1 h and (d) S. mutans after incubation at 37°C for 1 h. Reproduced from [112] with permission. Copyright The Royal Society of Chemistry, 2005... 

The main mechanism in all these methods is the physical separation and restriction of molecular mobility of the epoxy resin and the curing agent that are imposed by the solid state of the product. These adhesive systems generally provide a shelf life of up to 6 months at room temperature depending on the reactivity of the curing agent and resin. All these products require elevated temperatures to liquefy and crosslink. 

There are several analytical tools that provide methods of extrapolating test data. One of these tools is the Williams, Landel, Ferry (WLF) transformation.14 This method uses the principle that the work expended in deforming a flexible adhesive is a major component of the overall practical work of adhesion. The materials used as flexible adhesives are usually viscoelastic polymers. As such, the force of separation is highly dependent on their viscoelastic nature and is, therefore, rate- and temperature-dependent. Test data, taken as a function of rate and temperature, can be expressed in the form of master curves obtained by WLF transformation. This offers the possibility of studying adhesive behavior over a sufficient range of temperatures and rates for most practical applications. Fligh rates of strain may be simulated by testing at lower rates of strain and lower temperatures. 

It was thought in the past that the only mechanism for wall slip would be polymer desorption, i.e., an adhesive breakdown [25, 53]. However, lack of a strong temperature dependence would be inconsistent with an activation process of chain desorption. Since the onset of the flow discontinuity (i.e., stick-slip) transition was found to occur at about the same stress over a range of experimental temperatures, it was concluded from the outset [9] that the phenomena could not possibly have an interfacial origin. Thus, the idea of regarding the flow discontinuity as interfacial did not receive sufficient and convincing theoretical and experimental support in the past, not only because the transition was often accompanied by severe extrudate distortion and hysteresis, but also because the molecular mechanism for such an interfacial transition involving wall slip was elusive. 

Figure 7. The inverse temperature dependence of initial viscosity and direct dependence of cure chemorheology for poly(urea-urethane) adhesives yield activation energies of 9-12 Kcal/mole for viscous flow and 6-8 Kcal/mole for overall cure, respectively.

Nevertheless, such practical systems need to be investigated in terms of their adhesive strength and in particular in terms of their temperature dependence. 

TEMPERATURE DEPENDENCE OF ADHESIVE LAMINATES A PRACTICAL PROBLEM. 

The compressive strengths of the composites obtained increased and their temperature dependencies decreased with increasing fiber length, fiber-volume fraction, and density of the matrix foam. More specifically, the compressive strengtii of the composite was found to be proportional to that of the matrix and increased linearly with increased fiber-volume fraction in the experimental range employed (below 2% by volume). This result could be explained by Swift s sinusoidal model, assuming that the adhesion between fiber and matrix foam is perfect. 

The heat resistance of the adhesive layer is limited. Depending on the basic material of the adhesive, temperatures for continuous stress range between approximately 120 and 300 °C. 

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